In Japan, will sea dilute radiation pollution?

(CNN) -- I heard the news today, oh boy: The radioactive water leak at Japan's quake- and tsunami-stricken Fukushima Daiichi nuclear reactor has been stanched. It's the first anniversary of the fatal Massey Energy coal mine explosion. And we're days away from the anniversary of Deepwater Horizon's explosion and sinking and the start of last year's months-long blowout ordeal.

Quite a year for energy. And for people affected. For example, even if we dodged a bullet in the Gulf of Mexico last year and the effects of the oil did not come close to matching our fears of years-long disaster, the best smiley face one can honestly put on that event is that one season of idled tourism and fisheries cost billions.

So, what about Fukushima Daiichi? A bit like last year's Gulf blowout, we'll have to wait until the end of the crisis to begin calculating the aftermath. Fukushima is far from fixed, but the water leak has been stopped at present. So what about the radiation fallout's effects on sea life and seafood?

First, realize that it's not a matter of "radiation." We live surrounded by natural radiation. There is "radiation" in a banana. It's a matter of what kind of radiation and how much. Radioactive iodine, prevalent in water from Fukushima Daiichi's reactors, has a half-life of about eight days. That means that half of it has broken down and is no longer radioactive after about a week.

Other sources have much longer half-lives; radioactive cesium, also coming from the reactor, takes three decades to degrade by half. Complications abound; a low level in seawater can concentrate in seaweed, then move further into the food chain.

Radiation that may seem alarmingly high in water coming from the plant (definition of alarming: reports of more than 7.5 million times the legal limit of radioactivity, at its peak, according to The Telegraph) dissipates to much lower levels as it gets diluted in the massive amount of ocean water it enters. Fish, like tuna that migrate thousands of miles, would get fleeting exposure as they moved past the coast. Shellfish in the seabed locally would be more exposed.

What this means for the sea life itself is hard to judge, and legal limits mean little, because duration -- how long sea life is exposed -- means a lot. Dilution makes a big difference for sea life, compared with land animals.

According to Andrew Maidment, a professor of radiology at the University of Pennsylvania, after the 1986 Chernobyl disaster, wild boars within a 30-kilometer radius registered 470,000 becquerels per kilogram of cesium-137, (becquerels are units representing the number of nuclear disintegrations per second), while freshwater perch showed concentrations of 4,000 becquerels per kilogram. Eight years later, the levels were reduced to 5,000 becquerels per kilogram for the boars, and 200 becquerels per kilogram for the perch, Maidment said.

Take-home: Both water and time reduce wildlife's radioactivity exposure.

However, some birds around Chernobyl still show increased mutations and reduced survival. Could Japan's sea life also suffer genetic damage? Yes, but chances of that seem low at present.

Because of the vast volume of the Pacific Ocean, radioactive material is rapidly and effectively diluted, and what I've seen reported is consistent with the Food and Drug Administration's pronouncement that "fish and seafood are likely to be unaffected." Steam carrying radioactive contamination disperses in the air; where it lands depends on winds, and if it lands in the ocean, currents and diffusion can reduce its concentration to low levels.

As for seafood, there's essentially none being produced along the tsunami-devastated coast. What there is is subject to a lot of testing. The U.S. imports so little seafood from Japan that, generally speaking, there isn't much of a risk of coming into contact with seafood that's been affected by the radiation leaks.

Some higher-end sushi bars do get their fish from Japan, but they're likely to play it safe on this issue and get fish from elsewhere (or tell you they have; fibbing is a constant issue in the seafood business). The bigger issues with sushi is that top predators such as tuna already have relatively high mercury, and most are overfished and in decline.

Moreover, theoretical models do not predict that hazardous levels of radiation will reach the U.S. coast. However, the most scientifically accurate statement is that while any radiation can pose some health risk, in many cases, including this one, the human health risk is infinitesimally low. Driving a car is certainly much more dangerous.

Radioecologist F. Ward Whicker told National Geographic that the concentrations of iodine and cesium "would have to be orders of magnitude larger than the numbers I've seen to date to cause the kind of radiation doses to marine life that would cause mortality or reductions in reproductive potential. I am very doubtful that direct effects of radioactivity from the damaged reactors on marine life over a large area off the coast of Japan will be observed."

This is an unfolding story, and all above reassurances may be revised. Recently, for example, fish called lance were discovered contaminated with 526 becquerels per kilogram of radioactive cesium, slightly exceeding the legal limit of 500.

The opinions expressed in this commentary are solely those of Carl Safina.